Tree idea for Reading Area, tent... Options: attach some of the branches to the ceiling /or drape thin blue cloth for sky/night sky. Add white Christmas lights to the "sky" i.e. on the hanging fabric or wall ceiling.
My 3rd grade teacher did this. It was more of a rain forest theme and she had all us work on it, and when we did the rain forest unit we all had to make a specific animal. Part of our test was telling the class about the animal, what it does, and then we had to place it in the "correct part of the rain forest" we made together. It was so fun!